1. Field of the Invention
The invention relates to a liquid-crystal display device.
2. Description of the Related Art
For a liquid-crystal display device of the related art, for example, FIG. 13 showing a planar layout diagram and FIG. 14 showing a cross-section diagram taken along XIV-XIV in FIG. 13 illustrate a structure of a pixel section which is formed in an active-matrix substrate and provided with a storage capacitor element (also called a retentive capacitive element or auxiliary capacitive element).
In the examples of FIG. 13 and FIG. 14, a storage capacitor element 230 in the liquid-crystal display device has an inversely-staggered structure. First, a scan line 212 also functioning as a gate electrode of a thin-film transistor (TFT) is formed with low-resistance metal material such as aluminum on a glass substrate 211. Usually, from a productivity standpoint, a main wiring line 213 used for a storage capacitor element (CS) and the scan line 212 also functioning as the gate electrode of the thin-film transistor are simultaneously formed with a same metal material layer. A gate insulation layer 214 is formed on the gate electrode 212 (212G) and the main wiring line 213 used for a storage capacitor element (CS). The gate insulation layer 214 is formed with a silicon nitride layer. Then, an active element 215, a source wiring line 216, and a drain wiring line 217 are formed in the TFT section, and for the TFT section, a passivation layer 218, an overcoat layer 219, and a picture-element electrode 220 are formed. The source wiring line 216 and drain wiring line 217 are formed with a low-resistance metal material such as, for example, aluminum or aluminum alloy. The passivation layer 218 is formed with, for example, a silicon nitride layer, the overcoat layer 219 is formed with, for example, an acrylic resin, and the picture-element electrode 220 is formed with a transparent electrode (see, for example, Japanese Examined Patent Application Publication No. H01-33833).
Typically, the storage capacitor element (CS) includes the main wiring line 213, an intermediate electrode 221 formed by extending the drain electrode 217 from the TFT, and a gate insulation layer 214 formed between the main wiring line 213 and the intermediate electrode 221. The intermediate electrode 221 contacts the picture-element electrode 220 through a contact hole 222 formed in the passivation layer 218 and overcoat 219. In this case, since the main wiring line 213 and intermediate electrode 221 are respectively formed with low-resistance metal layers which are the same layers as the gate electrode 212 (212G) and drain wiring line 217, light from a backlight source is blocked and thus an aperture ratio decreases (see, for example, Japanese Unexamined Patent Application Publication No. H04-217230). When the intermediate electrode 221 is not used, a storage capacitor is directly formed between the picture-element electrode 220 and gate electrode 212 (212G). In this case, as previously described, the main wiring line 213 which is a low-resistance metal layer also makes an aperture ratio decrease.
In addition, as a method for improving viewing-angle characteristics, there is another method called “capacitive-coupling halftone grayscale method” (hereinafter referred to as “halftone method”).
In a Multidomain Vertical Alignment type or Twisted Nematic type liquid-crystal display device of the related art, there arises a phenomenon that its display screen becomes white-tinged when viewed from an oblique direction. When a voltage which is somewhat higher than a threshold voltage is applied to the picture-element electrode 220, a transmittance of the display screen viewed from an oblique direction becomes higher than that of the display screen viewed from an anterior direction. In addition, when the applied voltage becomes a certain higher level, a transmittance of the display screen viewed from an oblique direction becomes lower than that of the display screen viewed from an anterior direction. Therefore, small luminance-differences among a red pixel, a green pixel, and a blue pixel result in the aforementioned phenomenon that the display screen becomes white-tinged.
In the halftone method, as a countermeasure against the phenomenon, a pixel is divided into a plurality of sub-pixels which are capacitively coupled to one another. Since an electric potential is divided on the basis of a capacitance ratio of each sub-pixel, mutually-different voltages can be applied to the plurality of sub-pixels, respectively. Therefore, as a result, one pixel appears to have a plurality of areas which have different threshold voltages of transmittance vs. voltage characteristics (T-V characteristics). When, in this way, there are the plurality of areas in one pixel, which have different threshold voltages of the T-V characteristics, averaged T-V characteristics among these areas suppresses the phenomenon that a transmittance of the display screen viewed from an oblique direction becomes higher than that of the display screen viewed from an anterior direction. As a result, the phenomenon that the display screen becomes white-tinged when viewed from an oblique direction is also suppressed.
A structural example of the halftone method will be described with reference to FIG. 15 showing a planar layout diagram, FIG. 16A showing a cross-section diagram taken along XVIA-XVIA in FIG. 15, and FIG. 16B showing a cross-section diagram taken along XVIB-XVIB in FIG. 15.
As shown in FIGS. 15, 16A, and 16B, an outline of the structure is the same as that explained using FIG. 13. For example, a main wiring line 213 used for a storage capacitor element (CS) and the scan line 212 also functioning as a gate electrode of the thin-film transistor are simultaneously formed with the same metal material layer. A gate insulation layer 214 is formed on the gate electrode 212 (212G) and the main wiring line 213 used for a storage capacitor element (CS). The gate insulation layer 214 is formed with a silicon nitride layer. Then, an active element 215, a source wiring line 216, and a drain wiring line 217 are formed in the TFT section, and for the TFT section, a passivation layer 218 and a picture-element electrode 220 are formed. The source wiring line 216 and drain wiring line 217 are formed with a low-resistance metal material such as, for example, aluminum or aluminum alloy. The passivation layer 218 is formed with, for example, a silicon nitride layer, the overcoat layer 219 is formed with, for example, an acrylic resin, and the picture-element electrode 220 is formed with a transparent electrode. The picture-element electrode 220 is divided into a picture-element electrode 220A and a picture-element electrode 220B. The picture-element electrode 220B is connected to the drain wiring line 217 with the same structure as explained using FIG. 13. On the other hand, the picture-element electrode 220A is capacitively coupled to the drain wiring line 217. For the capacitive coupling, a control electrode 223 is formed in the middle of the drain wiring line 217, and an area of a certain size is arranged for determining a voltage value of each selected picture element. In the same way as previously mentioned, the drain wiring line 217 and control electrode 223, formed simultaneously with a low-resistance metal layer of same material, have a light-blocking effect and make an aperture ratio decrease.